Real Differences between OT and CRDT in Correctness and Complexity for Consistency Maintenance in Co-Editors

Abstract

OT (Operational Transformation) was invented for supporting real-time co-editors in the late 1980s and has evolved to become core techniques widely used in today's working co-editors and adopted in industrial products. CRDT (Commutative Replicated Data Type) for co-editors was first proposed around 2006, under the name of WOOT (WithOut Operational Transformation). Follow-up CRDT variations are commonly labeled as "post-OT" techniques capable of making concurrent operations natively commutative in co-editors. On top of that, CRDT solutions have made broad claims of superiority over OT solutions, and often portrayed OT as an incorrect and inefficient technique. Over one decade later, however, CRDT is rarely found in working co-editors; OT remains the choice for building the vast majority of today's co-editors. Contradictions between the reality and CRDT's purported advantages have been the source of much confusion and debate in co-editing researcher and developer communities. To seek truth from facts, we set out to conduct a comprehensive and critical review on representative OT and CRDT solutions and working co-editors based on them. From this work, we have made important discoveries about OT and CRDT, and revealed facts and evidences that refute CRDT claims over OT on all accounts. These discoveries help explain the underlying reasons for the choice between OT and CRDT in the real world. We report these results in a series of three articles. In this article (the second in the series), we reveal the differences between OT and CRDT in their basic approaches to realizing the same general transformation and how such differences had resulted in different technical challenges and consequential correctness and complexity issues. Moreover, we reveal hidden complexity and algorithmic flaws with representative CRDT solutions, and discuss common myths and facts related to correctness and complexity of OT and CRDT. We hope the discoveries from this work help clear up common myths and confusions surrounding OT and CRDT, and accelerate progress in co-editing technology for real world applications.

A semi-transparent selective undo algorithm for multi-user collaborative editors

“…In the literature, there are two highly researched concurrency control approaches for co-editing systems: operational transformation (OT) [42][43][44] and commutative replicated data type (CRDT) [45][46][47][48].…”

“…The CRDT-based solutions must convert the identifier-based operations to/from the position-based operations to finally achieve the consistency of shared and replicated documents. The time and space overhead of the CRDT-based solutions are dependent on the size of the object sequences because the conversion must update and search the object sequences [43].…”

“…According to the recent research studies [42][43][44], OT and CRDT both adopt the strategy of transforming operations to solve the consistency problems in collaborative applications. They are both a kind of transformation-based concurrency con-trol approaches but different in the way of transforming operations.…”

Towards the next generation of reactive model transformations on low-code platforms

“…• the concurrent editing of the target model from multiple EVM instances requires transactional model processing and locking mechanisms to guarantee its consistency, • alternatively, different lock-free mechanisms should be adopted for reactive transformations [40,41].…”

Real Differences between OT and CRDT under a General Transformation Framework for Consistency Maintenance in Co-Editors

“…The first CRDT solution for plain-text co-editing appeared around 2006 [40,41], under the name of WOOT (WithOut Operational Transformation). One motivation behind WOOT was to solve the FT (False Tie) puzzle in OT [53,55] (also discussed in detail in [73]), using a radically different approach from OT. Since then, numerous WOOT revisions (e.g.…”